1. Field of the Invention
The present invention relates to the field of the power semiconductor. It relates in particular to a gate turn-off thyristor for high reverse voltages, comprising
a p-type emitter layer, an n-type base layer, a p-type base layer and a cathode-side gate-cathode structure in a semiconductor substrate between an anode and a cathode; and PA0 a beveling at the substrate edge to reduce the upper surface field strength. PA0 the p-type base layer is subdivided into a central p-type base layer and a p-type base edge layer which surrounds the p-type base layer, PA0 the active part of the thyristor is disposed in the region of the central p-type base layer, PA0 the p-type base edge layer has a greater thickness than the central p-type base layer, PA0 the p-type base edge layer has a lower doping concentration than the central p-type base layer, PA0 the beveling is constructed as negative beveling, PA0 the negative beveling is disposed in the region of the p-type base edge layer, and PA0 cuts the pn junction formed by the p-type base edge layer and the n-type base layer. PA0 a first predeposition layer containing an accepter material with a high diffusion constant is introduced into the n-type doped semiconductor substrate on the cathode side in the edge region; PA0 a second predeposition layer containing an accepter material with a low diffusion constant is introduced into the n-type doped semiconductor substrate on the cathode side at least in the central region; PA0 in a subsequent diffusion step, the acceptors are simultaneously diffused into the semiconductor substrate from the two predeposition layers; and PA0 after the remaining structure of the thyristor has been produced, the negative beveling is provided at the substrate edge.
The invention relates furthermore to a process for producing such a gate turn-off thyristor.
2. Discussion of background
For a planar pn junction in a semiconductor component, for example a high-power thyristor, to be able to block high voltages (&gt;1000 V), special measures have to be taken in the edge region of the disk-shaped component substrate in order to reduce the increased field strength occurring at this point.
Known measures essentially comprise a lateral structuring of the doping zones, and specifically, either as external contouring by grinding off or etching out (positive, negative beveling; trench etching; mesa etching or substrate etching; in this connection see V.P. Sundersingh and A. A. Ghatol, Int. J. Electronics, Vol. 54, No.1 (1983), pages 127-137), or by introducing special doping structures (for example, in the form of so-called "guard" rings).
In the case of low-power gate turn-off thyristors, a simple trench edging which interrupts the otherwise planar pn junction between p-type base layer and n-type base layer has proved successful as edge contouring (on this point see: Brown Boveri Technik, 9 (1986), pages 519-525).
In the quite differently constructed field-controlled thyristors, the provision of a double trench which represents a combination of mesa and trench edging is furthermore known as edge contour (EP-A2-0,178,387). In order to provide sufficient space for the double trench, the very thin (5 .mu.m) and only slightly p-type doped (10.sup.16 cm.sup.-3) gate region merges at the edge into a deeper-drawn edge region which may be introduced into the semiconductor substrate, for example, by the selective aluminum diffusion known from EP-A2-0,216,954.
Hitherto no detailed solutions have been specified for the edge contouring of high-reverse-voltage GTO thyristors. Thus, the publication IEEE Trans. Electron Devices, vol. ED-28, No. 3 (1981), pages 270-274 has described only a GTO thyristor having a reverse voltage of 2500 V whose edges have been beveled by sandblasting. However, the said publication does not specify either the lateral doping profile of the p-type base layer nor does it deal with the type (positive or negative), shape and position of the said beveling.
A particularly simple, inexpensive solution which is compatible with pressure contacting is the wedge-shaped partial grinding of the slice edge at a small negative angle (the abovementioned negative beveling). However, an adequate reduction in the electric field strength at the edge is only possible with this technology if the beveling angle is either extremely small (=approx. 1.degree.) or the blocking pn junction is not too strongly asymmetrical (thickness of the pn junction &gt;70 .mu.m; doping concentration at the edge of the p-type profile &lt;5.times.10.sup.17 cm.sup.-3).
Applying negative beveling to high-reverse-voltage GTO thyristors with planar pn junction would therefore raise the following problem: the p-type base layer of these components has both to be highly doped (edge doping concentration N.sub.po &gt;10.sup.18 cm.sup.-3) and also have a small thickness or depth (penetration depth of the p-type base layer X.sub.Jp &lt;70 .mu.m).
An adequate reduction of the edge field strength would then only be achievable with the so-called small beveling angles. Such small beveling angles require, however, a wide edge region and consequently a loss of active area. In addition, they can be produced with the necessary precision only with difficulty.